1. Field of the Invention
This invention relates to vacuum interrupters, and, more particularly, to the design of electrode stems for axial magnetic field vacuum interrupters.
2. Description of the Prior Art
Vacuum interrupters are typically used, for instance, to reliably interrupt medium to high voltage a.c. currents of several thousands of amperes or more. They generally include a vacuum envelope enclosing a pair of facing contact electrodes that are relatively movable between a closed circuit position and an open circuit position. Each contact is connected to a current carrying electrode stem extending outside the vacuum envelope. Surrounding the contacts within the envelope is a vapor condensing shield aligned concentrically with the contacts and the electrode stems.
When the contacts are moved apart from the closed circuit position to the open circuit position, arcing of the current between the contacts occurs before the current is interrupted. The arcing can seriously damage the contacts, reducing the useful life of an interrupter. Metal from the contacts that is vaporized by the arc condenses back onto the contacts and also on the vapor shield, protecting the insulating vacuum envelope from accumulating deposits of metal.
The designs of practical commercial high-current vacuum interrupters have evolved over the past 30 years into two main types of contact arrangements, discussed in an article, entitled: The Vacuum Interrupter Contact. P. G. Slade, IEEE TRANS. ON COMPONENTS, HYBRIDS, AND MFG. TECH., V. MHT-7, No. 1, p. 25-32, March 1984, and included herein by reference. Each type of contact arrangement produces a magnetic field that helps to control the initially columnar arc and promote its transition to a diffuse mode. In a first type of contact arrangement, a magnetic field is impressed perpendicular to the arc column in a direction that forces the arc to move rapidly around the circular periphery of the contact surface. This can be accomplished using slotted-cup contacts or contacts having spiral-shaped arms, wherein the magnetic field is self generated by the a.c. current. In a second type of contact arrangement, an axial magnetic field is generated in the contact region that forces the high-current arc to rapidly become diffuse and continuously distributed within the contact gap.
The axial magnetic field for the latter type of vacuum interrupter is typically generated by field coils using the current in the interrupter. In a first type, internal structures, such as axial field coils typically assembled as parts of the arcing contacts, direct the currents so as to produce an axial magnetic field in the contact gap. In a second type, the current passes through a coil structure surrounding the exterior of the vacuum interrupter, and the axial magnetic field penetrates through the insulating wall of the device and into the gap region. In both cases, the axial magnetic field produced is proportional to the instantaneous current in the interrupter.
It is well known that the flow of sinusoidal current in the internal or external axial magnetic field coils induces eddy currents in the conducting structures within the axial magnetic field vacuum interrupters. These eddy currents are undesirable, since they act to reduce the magnitude of the axial magnetic field and increase its phase delay from the coil current. The prior art has concentrated on reducing eddy currents in the faces of the arcing contacts. For example, U.S. Pat. No. 3,946,179, to Murano, et al., described slotted designs for contacting face plates in axial magnetic field vacuum interrupters that would reduce the eddy currents in that part of the device. U.S. Pat. No. 4,935,588, to Hess, et al., described contacting face plates for axial magnetic field vacuum interrupters having slots filled with a material having a conductivity that is less than the conductivity of the major portion of the contacting face plate.
Eddy currents generated in the highly conductive electrode stems can have the deleterious effect of reducing the axial magnetic field in the contact gap and increasing its phase delay from the coil current. It is important to minimize the phase delay of the axial magnetic field so that the field will rise above a critical level for producing a diffuse arc for the greatest possible fraction of the current cycle. The eddy currents in the electrode stems also tend to heat the stems and thereby reduce their current carrying capacity.